PROJECT SUMMARY ADP-ribosylation is a posttranslational modification of proteins characterized by the addition of poly- ADP-ribose (PAR) in response to cellular stressors, such as excitotoxicity or oxidative stress.1 The PAR Polymerase (PARP) family catalyzes this reaction, which is reversed by two known factors, Poly-ADP Ribose Glycohydrolase (PARG) and ADP-ribosylhydrolase-like protein 2 (ADPRHL2).1 Using genome-wide linkage analysis and exome sequencing, inactivating mutations in ADPRHL2 have been identified in several large consanguineous families. The patients exhibit a pediatric onset neurodegenerative disorder with brain atrophy and sudden death from epilepsy. Previous studies have identified ADPRHL2 as the only active glycohydrolase present in mitochondria; however, its importance and role in the mitochondria remain understudied. The goal is to describe this clinical condition as a new syndrome cause of neurodegeneration and study oxidative-stress induced mechanisms by which loss of ADPRHL2 promotes cell death both in vitro and in vivo. The generation of iPSCs from patient fibroblasts, which can be further reprogrammed into a neural differentiation lineage, offers an exciting and relevant tool to test our model. First, the effect of the patient mutations on cellular and protein function in both patient neurons and CRISPR knockout mice mimicking these mutations will be determined. Second, the effects of loss of ADPRHL2 on mitochondrial function will be assessed with the help of an established collaboration with Dr. Gulcin Pekkurnaz's mitochondrial lab at UCSD. A collaboration with Dr. Gabriel Haddad's hypoxia fly lab at UCSD has already been established to generate a Drosophila model to test the evolutionary conservation of function of this gene. Third, pharmacological manipulation of the PARP pathway will be used to rescue the phenotypes both in vitro and in vivo in hopes of developing a treatment for this novel pediatric disease. The outcome of this work will not only provide novel insight into mitochondrial function, but will also identify potential treatments for a new early-onset neurodegenerative disease. Hypothesis: Mutations in ADPRHL2 lead to increased vulnerability to neuronal death in response to oxidative stress. Failed PAR hydrolysis due to absence of ADPRHL2 in the mitochondria leads to bioenergetic failure, ultimately triggering apoptosis. Aim 1: Test the hypothesis that ADPRHL2 patient mutations interfere with protein function and PARylation of mitochondrial proteins. Aim 2: Test the hypothesis that loss of ADPRHL2 leads to stress-induced defects in mitochondrial function and cell survival. Aim 3: Test the hypothesis that loss of ADPRHL2 can be pharmacologically or genetically rescued by inhibiting PARP activity in vitro and in vivo.